Apparatus for detection of a fire or of flames

ABSTRACT

Fire detecting apparatus includes two photoelectric devices, each having different spectral sensitivities. A difference signal corresponding to the difference between the output signals of the photoelectric devices is generated and an alarm signal is developed when the difference signal deviates by a predetermined amount from a predetermined value or range of values, depending upon application. A preferred embodiment also includes a delay means for delaying generation of the alarm signal for a predetermined time.

United States Patent 1191 Miiller APPARATUS FOR DETECTION OF A FIRE OR0F FLAMES Peter Miiller, Oetwil, Switzerland Cerberus AG., Mannedorf,Switzerland Filed: Dec. 1, 1970 Appl. No.: 94,113

Inventor:

Assignee:

Foreign Application Priority Data Dec. 3, 1969 US. Cl. IMO/228.2,250/83.3 H, 250/83.3 UV,

' 250/220, 356/222 Int. Cl. G08b 21/00 [58] Field of Search 340/228 R,228 S,

340/228.1, 228.2, 171; 250/83.3 UV, 83.3 R,

References Cited UNITED STATES PATENTS 6/1965 Vasec et a1. 340/228 R2,897,485 7/1959 Johnson 340/228 R Switzerland 18107/69 PrimaryExaminer-Donald J. Yusko Assistant Examiner-William M. WanniskyAttorney-Flynn & Frishauf [57] ABSTRACT Fire detecting apparatusincludes two photoelectric devices, each having different spectralsensitivities. A difference signal corresponding to the differencebetween the output signals of the photoelectric devices is generated andan alarm signal is developed when the difference signal deviates by apredetermined amount from a predetermined value or range of values,depending upon application. A preferred embodiment also includes a delaymeans for delaying generation of the alarm signal for a predeterminedtime.

23 Claims, 12 Drawing Figures 2 Sheets-Sheet 1 Fig. 6

Patented June 12, 1973 3,739,365

2 Sheets-Sheet 2 APPARATUS FOR DETECTION OF A FIRE OR OF FLAMES FIELD OFINVENTION The present invention relates to apparatus for thedetection ofa fire or of flames by means of emitted rays.

Apparatus of this type is preferentially utilized as a fire alarm or asa control unit for combustion installatrons.

BACKGROUND OF THE INVENTION It is already known that the presence of aflame may.

A known'device utilizes the typical flickering of flames, i.e. thevariation of intensity of the light radiation of the flame in a verylow-frequency oscillation zone, as the distinguishing feature of a flamevis-a-vis disturbance light radiation. In this known device theradiation strikes a photoelectric element whose output signal isconducted to a frequency-selective amplifier whose band-pass lies in theorder of magnitude between 5 and 25 Hz. The amplifier then feeds theamplified signal to a switching network. Even if the. frequencyband-pass of the amplifier optimally corresponds to the rate offlickering of flames, disturbances and false alarms are relativelycommon occurrences. If accidental variations of intensity in the ambientlight radiation lie in the same frequency zone, for example throughshadings or false flashes due to vibrating or slowly moving objects,false flashes of sunlight on water surfaces, flickering or waveringlight sources, etc., the a false alarm could be generated.

It has been attempted to eliminate the disturbance effect of externallight radiation sources by utilization of an infra-red sensitive photocell or by connecting of an infra-red filter which is especiallytranslucent for flame radiation in front of a photo cell. However, thisworks only if the infra-red radiation of the disturbance light source isvery small. With this construction a strong disturbance light sourcecould cause a false alarm.

Another known device utilizes the fact that aflame possesses arelatively large proportion of long-wave radiation (infra-red, forexample) and only a small pro portion of short-wave radiation (blue, forexample). This known device utilizes two different photoelectricdevices, for example photo resistances with different an alarm signalwhen the voltage at the junction point has a certain value. In a deviceof this type a false alarm can' be generated by constant infra-red lightradiation sources, such as heaters or heating ovens. On the other hand,with known device, an alarm signal can not be generated if a highintensity disturbance radiation in the short-wave zone ispresent. Thisapparatus is thus only conditionally utilizable, with the result that aDC. amplifier must be used whose operating point must be kept stable.This leads to complicated circuitry and additional expenditures.

A simple combination of the best features of the two known devicesdescribed above, namely the detection of the typically waveringintensity of light radiation of flames as well as of the relationbetween long-wave and short-wave radiation, is unfortunately notpossible. For example, the utilization of an AC. amplifier with theknown device using two photo resistances in series having differentspectral sensitivity would produce an inoperable system. This is becausein a flame, the red portion of the light radiation exhibits almost thesame intensity fluctuations in time as the blue portion, and theresulting red-blue fluctuation relation remains almost constant. Thus,at the junction point of the two photo resistances an almost constantA.C. potential without marked A.C. variation occurs.

It is therefore the main object of the present invention to provide areliable apparatus for flamedetection that will be substantially.uninfluenced by external disturbance light radiation sources.

SUMMARY OF THE INVENTION In accordance with the present invention, twophotoelectric devices, each having different spectral sensitivities areprovided. A difference signal corresponding to the difference betweenthe output signals of the photoelectric devices is generated, and analarm signal generator is responsive to the difference signal forgenerating an alarm signal when the difference signal deviates by apredetermined amount from apredetermined value.

A particularly effective embodiment of, the present invention comprisesat least one pair of photo elements having differentspectralsensitivities connected in series or in parallel with oppositepolarities. The-output of the photo element arrangement, which is thedifference between the output signals of the two photo elements, iscoupled to theinput of an analyzerthat is sensitive over a limitedlow-frequency AQC. range.- The analyzer includes a discriminator circuitwhich emits an alarm signal when theoutput signal of the analyzerdeviates by a predetermined amount from a predetermined value.

In a particularly suitable embodiment, the spectral sensitivities of thetwo photo elements are varied in such a waythat for predetermineddisturbance light radiation, the difference .of the output signals ofthe two photo elements is smaller (i.e., by at least a factor of 10)than theindividual signals, that is, the difference signal isessentially zero.

DRAWINGS FIG. 1 is aschematic representation of a device in accordancewith the present invention;

FIG. 2 illustrated a circuit for passive photo elements with twooppositelyconnected switching networks;

FIG. 3 illustrates a circuit for passive photoelements with twooppositely connected switching networks and a common direct-currentsupply;

FIG. 4 illustrates a circuit for active photoelements;

FIG. 5 illustrates a circuit for active photoelements with a commontuning potentiometer;

FIG. 6 illustrates a circuit for currentemitting photoelements;

FIG. 7 illustrates a dual photoelement;

FIG. 8 illustrates two photoelements with sensitivities that arevariable by means of a common screen or light shield;

FIG. 9 illustrates a device comprising two photoelements with reflectionfilters;

FIG. 10 illustrates a device comprising two photoelements with a commondichroic filter;

FIG. 11 illustrates a circuit for the evaluation of the signals of thephotoelectric; and

FIG. 12 illustrates a device with more than two photoelements.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 represents schematically afire detection device-in accordance with the present invention. The redlight rays r and blue light rays b emitted from flame 1 simultaneouslystrike photoelectric cells 2 and 3, respectively. The term photoelectriccells is understood to mean any device which under the action of lightradiation changes its electrical characteristics. Examples of activephotoelements are selenium cells, silicon cells, solar cells, etc.Examples of passive photoelements are gas-filled or vacuum photo cells,photo diodes, photo resistances, etc. Photoelectric cells 2 and .3 havea different spectral sensitivity. Cell 2 is responsive to red light andcell 3 to blue light. This can for example be effected either in thatthe photosensitive layers of the cells consist of different materials,in that filters of different spectral permeability r or b are placed inthe path of the light rays, or in that reflection filters with differentspectral reflection are used for the two cells. The two photoelectricdevices emit coherent electric signals, for example, voltages orcurrents of differing intensity depending upon impinging light. Theintensity of the two signals of cells 2 and 3 can be tuned (or varied)in different ways independent of each other, such as by mounting amechanical screen in front of the photoelements, by using variousballast resistors connected to the photoelements, or by using additionalamplifiers and other circuit devices.

The output signals of the photoelectric devices are conducted to adevice 4, which generates an output signal which is a function of thedifference between the two signals from cells 2 and 3.'

This difference signal is conducted to a band pass filter 5, whichpasses only the A.C. portion of the difference signal which lies in aspecified frequency range. The frequency range between 2 and 50 Hz hasbeen found to be particularly suitable in practice. If still betterselectivity of the flame radiation as against disturbance lightradiation is desired, this frequency range (i.e. the band pass of filter5) may be even more narrowly restricted, for example, to the rangebetween 5 and 25 Hz.

The out put of band pass filter 5 is conducted to an amplifier 6, theoutput of which is conducted to a discriminator 7. Discriminator 7generates an alarm signal which is fed to an alarm or control device 8if the incoming signal thereto exceeds or falls short of certainpredetermined values. Discriminator 7 is preferably a circuit whichemits an output alarm signal if the input signal thereto deviatespositively or negatively by a certain amount from a fixed value, or ineither direction by a certain amount from zero. Alternatively, an alarmsignal can be fed to alarm device 8 if the effective value or anotherappropriate mean value of the output signal from amplifier 6 exceeds agiven threshold value.

The sensitivities of the photoelectric devices (i.e., cells 2 and 3) canbe tuned (or varied) as described above, in such a way that for adisturbance light radiation that occurs particularly often, i.e. forsunlight or for especially strong light sources in the vicinity of themonitoring apparatus, the output-signals of the photo electric devicesfor a radiation of this spectral composition would be equal. Thus, thedifference signal becomes zero, and the discriminator circuit 7 will inthis case under the action of such a disturbance radiation emit no alarmor control signal. With all other light radiations having differentspectral compositions the difference of the electrical output signals ofphotoelectric devices 2 and 3 will not be zero, but will deviate fromzero in one direction or the other.-The discriminator circuit will inthis case emit an alarm signal to be fed to claim device 8. In thismanner the screening-out of certain known disturbance light radiationswhich would normally cause a false alarm is easily effected withoutgreat-expense.

The discriminator circuit 7 can also be designed such that an alarmsignal is emitted only when the difference signal from circuit 4deviates from zero with a certain predetermined polarity. Thus, adisturbance light radiation of quite specific spectral composition whichwould normally cause a false alarm may be easily and completely screenedout, and that beyond this, an alarm can be set off only if, in the lightradiation striking the photocells, the longwave portion is preponderant.In special cases, the system can be such that an alarm will be generatedwhen the shortwave portion is preponderant, for example with fire alarmswhich react only to the ultraviolet light radiation of a flame.

In addition, the discriminatorcircuit 7 can also contain an integrator(or other appropriate delay means) so that an alarm will not be set offimmediately upon receiving short voltage impulses, butonly when the ex-.ceeding of the predetermined values exists forv a specified length oftime. In this manner short duration disturbances, through voltageimpulses of short duration, will not cause generation of a false alarm.

Further, the discriminator circuit 7 may include a locking or latchingcircuit such that upon the setting off of an alarm, the alarmautomatically holds in its on" condition, and can be re-set from acentral station. This can be. easily effected by connecting a latchingrelay, bistable multivibrator, or the like, to the output of thediscriminator as is well known in the art. The actuating of the alarmdevice can also be indicated by an optical indicator device 9 such as alight, which is installed either in the flame detection unit itself orin the alarm central control station and can serve for the localization(i.e., identification) of an actuated alarm unit. Optical indicator 9can be connected to alarm device 8 (as shown) or to discriminator 7.

' The sequence of the steps of generation of the difference signal,frequency filtering and amplification can be changed at will. Naturallythe different circuits can be comprised single elements, as shown, or bycombined special devices. For example, the generation of the differencesignal can be accomplished by corresponding coupling of thephotoelectric devices. The band pass filter amplifier and discriminatorcan be comprised in a single analyzer unit, which may also include acircuit for generating the difference signal (or its equivalent).

For the connection of the device of the present invention with a centralalarm station, known circuits for fire alarm systems may be utilized.For example, a device for function-monitoring of the system may also beprovided whereby a signal is emitted from the central station to producealarm-simulating conditions in the device so that the device is enabledto emit an alarm signal that can be registered in the central station.Such function-monitoring can be carried out in a known fashion throughdigital analysis, through logical circuits or through additional A.C.signals. Throughout the drawings, the same reference numerals are usedto designate the same or similar elements.

FIG. 2 illustrates anarrangement-in which two photoelectric devices 11and 12 are connected together in a differential circuit, which isconnected to an analyzer 10. The photoelectric devices 11 and 12 eachcomprise a passive photoelement 13, for example a photo resistance or aphoto diode, a ballast resistor 14 and a battery 15. Any suitable D.C.source can be used in place of batteries 15. In front of thephotoelement 13 is a filter 16 having a certain spectral permeability.The pho' toelectric device 12 differs from device 11 only in that thefilters 16 possess different spectral permeabilities. Device 11 is madesensitive to red light and device 12 is made sensitive to blue light, asis indicated in FIG. 2. The potential drop at ballast resistor 14 servesas an output signal of the photoelectric mechanism. The two devices 11and 1 2 are now connected with each other at either end of the ballastresistors 14 in such a way that the respective potential drops at thetwo ballast resistors have opposite polarities. The resistors 14 areconnected with the analyzer via leads 17 and 18. The signal AU appearingacross leads 17 and 18 is the difference AU of the voltage at the twoballast resistors 14, and thus the difference of the output signals ofthe two photoelectric devices 11 and 12. The analyzer 10 then combinesthe functions of circuits 5, 6 and 7 of FIG. 1.

active photoelements needs no voltage source and the photoelectricdevices 11 and 12 can, in its simplest form, consist only of aphotoelement 13 and a ballast resistor 14. The photoelements are againconnected with opposite polarity so that at leads l7 and 18 from theballast resistors, once more the difference of the output signals existsand can be conducted to analyzer 10.

Additionally, on one of the ballast resistors 14, a reference potentialmay be tapped off and fed to analyzer 10 via lead 19. The referencepotential can be used in the analyzer 10 to detect the sense (orpolarity) of the difference of the output signals of the photoelectricde- 'vices. In this manner, not only a specified disturbance radiationcan be prevented from generating false alarms, but additionally thealarm is set off only when the radiation becomes preponderant in aspecified portion of the light spectrum.

In FIG. 5 two networks with active photoelements 13 with differingspectral sensitivity as in FIG. 4 are connected together. However, theballast resistors for the two photoelements are made up of a singlecommon potentiometer 20 with a variable tap. One portion of theresistance of potentiometer 20 serves as the ballast for circuit 11, theremaining portion serving as a ballast resistor for circuit 12. By meansof the variable tap, the

FIG. 3 illustrates a circuit in which two photoelectric 14 once morehave opposite polarity, so that again the.

signal AU representing the difference of the output signals of devices11 and 12, is conducted to analyzer 10. With this arrangement, thegreater part of the output signals of both photoelectric devices can bemodulated independently of each other and optimal tuning of responsecharacteristics, to remove sensitivity to a specified disturbanceradiation may be obtained.

FIG. 4 illustrates two photoelectric devices 11 and 12,- which compriseactive photoelements 13. To this end selenium, silicon, or solar cellsmay be used, or any other type of photoelectric cell which gives avoltage or a current in response to light. The coupling network forrelation of the two resistance portions and also thesensitivity-relation of the two devices 11 and 12, may be varied andtuned to eliminate sensitivity to a particular disturbance radiation.Instead of voltage, current can also serve as output signal of thephotoelectric mechanism.

In FIG. 6 there are two active current-sending photoelements 11 and 12,for example selenium or silicon cells with differing spectralsensitivty, connected in parallel with each other and in parallel withanalyzer 10. The current difference A1 of the two photoelements 11 and12 then flows into the conductors and analyzer 10. In this case,analyzer 10 must be modified to have a small input resistance relativeto the internal resistance of photoelements l0 and 11, to sense thecurrent AI.

FIG. 7 illustrates an arrangement of two photoelectric elements in theform of photo-resistances, on a common base material 21.This constitutesa dual element whose two halves 22 and 23 have the same characteristics.The two photoelectric elements 22 and 23 are covered by optical filters.24 and 25, which, however, pass different frequency ranges of the lightradiation spectrum. Such an arrangement has the advantage that in use,both photoelements are impinged by very nearly an equal light radiationintensity. Alternatively dual cells with layers 22 and 23 of differentspectral sensitivity can be used. The two layers 22 and 23 can bearranged on top of each other, whereby the top layer is permeable forthe radiation for which the lower layer is sensitive.

FIG. 8 shows an example of a mechanism for adjustment of the effectivecharacteristics of two photoelements 27 and 28. A mechanically movablescreening device 26, such as a diaphragm, damping filter or othermaterial for either blocking, reducing or otherwise changing lighttransmission characteristics, can be moved in such a way that one orboth of the photoelectric devices 27 and 28 may be partially screened.

FIG. 9 illustrates an arrangement in which the incoming light radiationis impinged on two reflection filters 29 and 30 with different spectralreflection characteristics, the light being then conducted ontophotoelements 27 and 28. This is equivalent to various otherarrangements described above. In FIG. 10 a dichroic fil-- ter 31 ismounted in the path of incoming light radiation. Filter 31 reflects onlythe portion of the radiation having a specified spectral composition onto a photoelement 27. Filter 31 passes another portion of the radiationwith differing spectral composition therethrough onto photoelement 28.With this arrangement, exactly equal radiation for both photoelements 27and 28 is obtained. As a dichroic filter 31 may be used very tyin metallayers, for example gold and copper or transparent optical layers whosethickness lies in the order of magnitude of the light-wave lengths, aswell as combinations of such layers with different refractive index(which have recently become known as cold-light mirrors, warm-lightmirrors or interference filters).

FIG. 11 illustrates the circuitry of an analyzer 10. Input leads 17 and18 are the leads shown, for example, in FIGS. 2 to 6. The differencesignal'of two photoelectric devices is conducted to analyzer over leads17 and 18. Thus, in this embodiment, a circuit (such as circuit 4 ofFIG. 1) for generating the differential signal in this case is notnecessary. The difference signal, conducted over terminals 17 and 18 tothe analyzer 10, is fed through aninput capacitor 32 to a firsttransistorized amplifier stage 33. Capacitor 34 on the output ofamplifier stage 33 serves to limit the high frequencies. The output 35of the first amplifier stage 33 is coupled to further amplifier stages(not shown) and then to the discriminator circuit. The discriminatorincludes the two rectifiers 36 and 37 which serve for rectification andsignal doubling; and capacitor 38, its charging resistance 39 and itsbleeder resistor 40, which serve as an integration stage with aspecified time constant, i.e. for the time lag of the discriminator.Once the charge on capacitor 38 reaches a specified value, break-down(Zener) diode 41 which is coupled to the output-of the integration stagebecomes conductive and turns on controlled rectifier (i.e., SCR) 42.This causes a signal to be fed to alarm-lead 43, thereby actuating analarm device 44 which can give off an acoustic or an optical signal, orwhich controls an appropriate switching operation. In this embodiment,analyzer 10 includes also a control or alarm device 8 of FIG. 1. Thecontrolled rectifier 42 is connected such that once turned on, itremains turned on, even when the input signal falls below the actuatingthreshold value as determined by break-down diode 41. Controlledrectifier 42 can be turned off by means of circuit breaker 45 whicheffectively opens lead 43. An optical indicator device 46 is connectedinto the switching network of the controlled rectifier 42; this permitsvisual recognition of the actuating state of the controlled rectifier 42and of the alarm device.

Naturally for a flameor fire-detection device of the type describedabove any and all other circuits known in the art may be utilized, aslong as they serve the same function to carry out the present inventiveconcepts. Instead of working with transistors and semiconductors thecircuit can also be fabricated with vacuum tubes and instead of acontrolled rectifier, an ionical relay, for example a cold cathodevalve, can be utilized, which can simultaneously serve as a visualindicator for the state of the circuit instead of using a-separateindicator device.

Likewise, other known discriminator circuits can be used, for example,those that generate the effective value of the signal or those thatgenerate an alarm sig-- nal when the instantaneous value of the AC.signal is exceeded in a predetermined direction. Also, digitaldiscriminators may be used which, for example, when a predeterminedlimit is exceeded, generate an impulse and only give out an alarm signalif a specified number of impulses have been generated within apredetermined period of time.

The input amplifier stages of analyzer 10 must have input impedanceswhich are compatible with the impedances of the photoelectric devices.

Furthermore, it is not necessary that only two photoelectric devices beused in the present invention. To provide a greater input signal to theanalyzer, a greater number of photoelectric devices can be coupledtogether such that the outputs of all devices having one spectralsensitivity are additively combined, and the outputs of all deviceshaving the other spectral sensitivity are additively combined. Also,devices of equal spectral sensitivity can be grouped in a unit, or also,devices of differing sensitivity can be connected alternatively inseries. Further, pairs of oppositely connected photoelectric devices ofdiffering sensitivity can be connected in series in such a manner thateach pair is sensitive for radiation coming from a specified direction.In this manner, fireor flame-alarm device with good peripheralsensitivity may be constructed.

FIG. 12 represents such a peripherally sensitive device with four pairsof photoelectric devices. Each pair contains two active photoelements47, in front of which a red filter 48 or a blue filter 49 is mounted asshown in FIG. 12. The pairs can naturally also be set up asdual-photoelements. For such pairs are connected in series such thateach pair is sensitive only to radiation from a given direction. Theends of the series connection of pairs of elements are conducted overleads 17 and 18 to an analyzer, which is similar to analyzer l0discussed hereinabove. The operation of FIG. 12 should be apparent.

I claim: 1 1. Apparatus for detecting fire or flames in the presence ofdisturbing radiation of a predetermined spectral composition resultingfrom ambient light which should not give an alarm comprising:

first and second photoelectric devices, each having different spectralsensitivities with respect to different spectral ranges with maximumresponse, each in the different spectral ranges and producing respectiveoutput signals; electrical circuit means including said photoelectricdevices for generating a difference signal corresponding to thedifference between the output signals of said first photoelectricdevices and the output signals of said second photoelectric devices,

said difference signal being essentially zero for the disturbingradiation and having an a-c component in a low-frequency range of aboutbetween 2-50 Hz differing from zero'in the presence of, and due to theflicker of flames;

means sensing the a-c component in said lowfrequency range only, of saiddifference signal; and alarm generating means responsive to the senseda-c component of the difference signal in said lowfrequency range onlyfor generating an alarm signal when the a-c component of said differencesignal in said low-frequency range exceeds a predetermined value.

2. Apparatus according to claim 1, wherein the low frequency range ofthe a-c component to which the alarm signal generating means isresponsive lies between and 25 Hz.

3. Apparatus according to claim 1 wherein said alarm signal generatingmeans includes:

filter means for passing only signals corresponding to said differencesignal within a predetermined frequency range; and

a discriminator means coupled to the output of said filter means forgenerating said alarm signal when said difference signal deviates by apredetermined amount from said predetermined value.

4. Apparatus according to claim 1 wherein said alarm signal generatingmeans generates said alarm signal when the magnitude of said differencesignal deviates by a predetermined amount from zero.

5. Apparatus according to claim 1 wherein said alarm signal generatingmeans generates said alarm signal when said difference signal deviatesby a predetermined amount with a predetermined polarity from zero.

6. Apparatus according to claim 1 wherein said alarm signal generatingmeans includes means for delaying for a predetermined period of timegeneration of said alarm signal when said difference signal exceeds thepredetermined value by.

7. Apparatus according to claim 1 comprising means for changing at leastone of the spectral sensitivity and the amplification of at least one ofsaid photoelectric devices such that the difference of the outputsignals of said photoelectric devices for the disturbance lightradiation with a pre-specified spectral composition is much smaller thanthe output signals of each individual photoelectric'device.

8. Apparatus according to claim 7 wherein said difference signals forsaid disturbance light radiation with said prespecified spectralcomposition is smaller by at least a factor of 10 than said outputsignals of each individual photoelectric device.

9. Apparatus according to claim 1 wherein said photoelectric devices arecoupled together in series opposing relation and said difference signalgenerating means comprises means coupling the free terminal of saiddevices to said alarm signal generating means.

10. Apparatus according to claim 1 wherein said difference signalgenerating means comprises means coupling said photoelectric devicestogether in parallel opposing relation and means coupling said parallelconnected devices to said alarm signal generating means.

11. Apparatus according to claim 1 wherein each of said photoelectricdevices comprise at least one active photoelectric element and a ballastresistance coupled thereto, the output signal of the device being thepotential drop at said ballast resistance or part thereof.

12. Apparatus according to claim 1, wherein each of said photoelectricdevices comprise at least one passive photoelement, a ballast resistancecoupled thereto, and a direct-current supply coupled thereto, the outputsignal of the device being the potential drop at said ballast resistanceor part thereof.

13. Apparatus according to claim 12 wherein said photoelectric devicesare opposingly connected together and to a common direct-current supply.

14. Apparatus according to claim 13 comprising a potentiometer coupledas a common ballast resistor to said photo-electric devices such thatone portion of said potentiometer is a ballast resistor of one of saidphotoelectric device and that another portion of said potentiometer is aballast resistor of the other photoelectric device.

15. Apparatus according to claim 1 wherein said photoelectric devicesinclude respective photoelements having different-type photosensitivelayers with differing spectral sensitivities.

16. Apparatus according to claim 1 wherein said photoelectric devicesinclude respective photoelements having photosensitive layers of thesame type and respective filters having differing spectral permeabilityor reflection characteristics mounted in the path of the light impingingon said photoelements.

17. Apparatus according to claim 1 comprising means for varying thelight impinging on at least one of said photoelectric devices.

18. Apparatus according to claim 1 wherein said photoelectric devicesinclude respective photoelements oil" a common base-material, and meansfor causing said photoelements to exhibit a differing spectralsensitivity.

19. Apparatus according to claim 1 comprising a dichroic filterconnected in front of both photoelectric devices which are arranged suchthat the portion of the radiation passed by said dichroic filter strikesone pho toelectric element, while the portion of the radiation reflectedby said dichroic filter strikes the other photoelectric device.

20. Apparatus according to claim 1 comprising a plurality of pairs ofoppositely connected photoelectric devices, each of said pairs beingconnected in series relative to one another and arranged such that eachpair receives light radiation from a different direction.

21. Apparatus according to claim 1, comprising means for maintainingsaid alarm signal in its on condition when an alarm signal has beengenerated.

22. Apparatus according to claim 3 wherein said discriminator meanscomprises means for automatically maintaining said alarm signal in itson condition when an alarm signal has been generated.

23. Apparatus according to claim 1 wherein said alarm signal includes anoptical signal for visually indicating said alarm signal.

1. Apparatus for detecting fire or flames in the presence of disturbingradiation of a predetermined spectral composition resulting from ambientlight which should not give an alarm comprising: first and secondphotoelectric devices, each having different spectral sensItivities withrespect to different spectral ranges with maximum response, each in thedifferent spectral ranges and producing respective output signals;electrical circuit means including said photoelectric devices forgenerating a difference signal corresponding to the difference betweenthe output signals of said first photoelectric devices and the outputsignals of said second photoelectric devices, said difference signalbeing essentially zero for the disturbing radiation and having an a-ccomponent in a low-frequency range of about between 2-50 Hz differingfrom zero in the presence of, and due to the flicker of flames; meanssensing the a-c component in said low-frequency range only, of saiddifference signal; and alarm generating means responsive to the senseda-c component of the difference signal in said low-frequency range onlyfor generating an alarm signal when the a-c component of said differencesignal in said low-frequency range exceeds a predetermined value. 2.Apparatus according to claim 1, wherein the low frequency range of thea-c component to which the alarm signal generating means is responsivelies between 5 and 25 Hz.
 3. Apparatus according to claim 1 wherein saidalarm signal generating means includes: filter means for passing onlysignals corresponding to said difference signal within a predeterminedfrequency range; and a discriminator means coupled to the output of saidfilter means for generating said alarm signal when said differencesignal deviates by a predetermined amount from said predetermined value.4. Apparatus according to claim 1 wherein said alarm signal generatingmeans generates said alarm signal when the magnitude of said differencesignal deviates by a predetermined amount from zero.
 5. Apparatusaccording to claim 1 wherein said alarm signal generating meansgenerates said alarm signal when said difference signal deviates by apredetermined amount with a predetermined polarity from zero. 6.Apparatus according to claim 1 wherein said alarm signal generatingmeans includes means for delaying for a predetermined period of timegeneration of said alarm signal when said difference signal exceeds thepredetermined value by.
 7. Apparatus according to claim 1 comprisingmeans for changing at least one of the spectral sensitivity and theamplification of at least one of said photoelectric devices such thatthe difference of the output signals of said photoelectric devices forthe disturbance light radiation with a pre-specified spectralcomposition is much smaller than the output signals of each individualphotoelectric device.
 8. Apparatus according to claim 7 wherein saiddifference signals for said disturbance light radiation with saidprespecified spectral composition is smaller by at least a factor of 10than said output signals of each individual photoelectric device. 9.Apparatus according to claim 1 wherein said photoelectric devices arecoupled together in series opposing relation and said difference signalgenerating means comprises means coupling the free terminal of saiddevices to said alarm signal generating means.
 10. Apparatus accordingto claim 1 wherein said difference signal generating means comprisesmeans coupling said photoelectric devices together in parallel opposingrelation and means coupling said parallel connected devices to saidalarm signal generating means.
 11. Apparatus according to claim 1wherein each of said photoelectric devices comprise at least one activephotoelectric element and a ballast resistance coupled thereto, theoutput signal of the device being the potential drop at said ballastresistance or part thereof.
 12. Apparatus according to claim 1, whereineach of said photoelectric devices comprise at least one passivephotoelement, a ballast resistance coupled thereto, and a direct-currentsupply coupled thereto, the output signal of the device being thepotential drop at said ballast resistance or part thereof.
 13. Apparatusaccording to claim 12 wherein said photoelectric devices are opposinglyconnected together and to a common direct-current supply.
 14. Apparatusaccording to claim 13 comprising a potentiometer coupled as a commonballast resistor to said photo-electric devices such that one portion ofsaid potentiometer is a ballast resistor of one of said photoelectricdevice and that another portion of said potentiometer is a ballastresistor of the other photoelectric device.
 15. Apparatus according toclaim 1 wherein said photoelectric devices include respectivephotoelements having different-type photosensitive layers with differingspectral sensitivities.
 16. Apparatus according to claim 1 wherein saidphotoelectric devices include respective photoelements havingphotosensitive layers of the same type and respective filters havingdiffering spectral permeability or reflection characteristics mounted inthe path of the light impinging on said photoelements.
 17. Apparatusaccording to claim 1 comprising means for varying the light impinging onat least one of said photoelectric devices.
 18. Apparatus according toclaim 1 wherein said photoelectric devices include respectivephotoelements on a common base-material, and means for causing saidphotoelements to exhibit a differing spectral sensitivity.
 19. Apparatusaccording to claim 1 comprising a dichroic filter connected in front ofboth photoelectric devices which are arranged such that the portion ofthe radiation passed by said dichroic filter strikes one photoelectricelement, while the portion of the radiation reflected by said dichroicfilter strikes the other photoelectric device.
 20. Apparatus accordingto claim 1 comprising a plurality of pairs of oppositely connectedphotoelectric devices, each of said pairs being connected in seriesrelative to one another and arranged such that each pair receives lightradiation from a different direction.
 21. Apparatus according to claim1, comprising means for maintaining said alarm signal in its oncondition when an alarm signal has been generated.
 22. Apparatusaccording to claim 3 wherein said discriminator means comprises meansfor automatically maintaining said alarm signal in its on condition whenan alarm signal has been generated.
 23. Apparatus according to claim 1wherein said alarm signal includes an optical signal for visuallyindicating said alarm signal.